This invention relates to a data pulse receiver arrangement of a type suitable for the acquisition of data pulsed which occur in a serial bit stream in a received information signal in which one level of the signal (e.g. high) represents a binary value `1` and another of the signal (e.g. low) represents a binary value `0`, said arrangement including a data clock pulse generator for clocking the data pulses into the data pulse receiver arrangement. The invention relates more particularly to a phase shifter in or for use in such data clock pulse generator.
A data pulse receiver arrangement of the above type (which is known for instance from Mullard Technical Information No. 34, dated September 1976, and Mullard Technical Information No. 54, dated August 1977) has application in data transmission systems in which data transmission and reception is not synchronized. Such a data transmission system, is for example, the BBC/IBA Teletext television transmission system in which coded data pulses representing alphanumeric text or other message information are transmitted in a video signal in at least one television line in field-blanking intervals where no picture signals representing normal picture information are present. United Kingdom Patent Specification No. 1,370,535 discloses a television transmission system of this form.
A difficulty that occurs with such an application of the data pulse receiver arrangement is to synchronizing clock pulses locally generated therein with the received data pulses. One technique for achieving this synchronization is to generate a local data pulse clock independently of the received data pulses and then shift its phase into synchronism with them. A data clock pulse generator which is suitable for this purpose may comprise an oscillator arranged for oscillation at a predetermined frequency to produce locally generated clock pulses, together with a phase sensitive detector which is operable to produce a control signal in accordance with the relative phases of the received data pulses and the clock pulses, which control signal is then used to correct the phase of the oscillator output signal to bring the data pulses and clock pulses into synchronism.
The control exercised by the control signal may act directly on the oscillator (e.g. in the case of a voltage-controlled oscillator), so that the phase of the oscillator output signal, as actually produced, is corrected. Alternatively, the control exercised by the control signal may cause a phase shifter to alter the phase of the oscillator output signal after it has been produced. This latter form of control permits a highly stable oscillator, such as a crystal-controlled oscillator, to be used to generate the clock pulses, but it requires the use of a phase shifter which can produce a large accurate phase shift.
In U.S. Pat. No. 3,475,626, there is described a phase shifter which can phase shift a single frequency signal accurately by a desired amount in the range 0.degree. to 360.degree.
This phase shifter is of a character comprising phase quadrature means for producing, in phase quadrature, first and second signal versions of the single frequency signal, function generator means responsive to a control signal representative of a required phase shift to produce first and second control factors, multiplier means operable to multiply said first and second signal versions by said first and second control factors, respectively, to produce resultant first and second signal versions in phase quadrature having respective controlled amplitudes, and combining means for combining said first and second resultant signal versions to produce an output signal which is a phase shifted version of said (original) single frequency signal, the controlled amplitudes of said resultant signal versions determining the phase of the output signal.
This phase shifter may be so arranged that with said resultant first and second signal versions in phase quadrature expressed as x cos wt and y sin wt, where x and y are their respective controlled amplitudes, the relationship .sqroot.x.sup.2 +y.sup.2 =A, where A is the output signal amplitude, is achieved to provide a substantially constant amplitude for A. The output signal (x cos wt+y sin wt) can be expressed as A sin (wt+.theta.), where .theta. is the required phase shift.
The requirement for the phase shift (.theta.) to lie anywhere between 0.degree. and 360.degree. can be achieved by so arranging the function generator means so that the controlled amplitudes (x and y) are made positive or negative, selectively, so that .theta. lies in one of the four quadrants 0.degree. to 90.degree., to 180.degree., 181.degree. to 270.degree. and 271.degree. to 360.degree..